Any electronic device that detects and/or processes optical signals must convert the sensed signals to electrical signals. This is accomplished with a photodetector. Image sensors include a spatial arrangement of photodetectors (pixels) that can be used to record and reconstruct an image. Image sensors are used in a wide variety of applications, e.g., toys, games, cameras, medical equipment, security equipment, process monitoring, portable handsets, personal digital assistants, scientific instruments, etc. The modern types of image sensors include charge coupled devices (CCDs) and CMOS (Complementary Metal Oxide Semiconductor) image sensors (also referred to as active pixel sensors).
A CCD sensor includes an array of linked, or coupled, light-sensitive capacitors. A CCD gets its name from the way the charges on its pixels are read after an exposure. After the exposure the charges on the first row are transferred to a place on the sensor called the read out register. From there, the signals are fed to an amplifier and then on to an analog-to-digital converter. The CCD shifts one whole row at a time into the readout register. The readout register then shifts one pixel at a time to the output amplifier.
CMOS image sensors are fabricated on semiconductor substrates, using the CMOS fabrication process used to manufacture computer processors and memories. Pixels in CMOS image sensors have their own charge-to-voltage conversion. In a typical CMOS pixel there is a photodetector, typically a photodiode or photogate, and a number of transistor devices. The photodetector can be reset when it is effectively connected to the power supply through a reset transistor. Another transistor typically acts as a buffer and allows the pixel voltage to be observed without removing the accumulated charge. A row-select transistor is a switch that allows a single row of the pixel array to be read by read-out electronics. In a typical CMOS image sensor, the pixels are arranged in a two-dimensional row and column arrangement. Pixels in a given row share reset lines permitting a row to be reset. Pixels are also selected by row. Outputs of each pixel in any given column are tied together. As one row is selected at a time no competition for the output line occurs. Further amplifier circuitry is typically on a column basis. The CMOS image sensor itself typically includes integrated circuits, e.g., column amplifier circuitry and read out electronics, which permit the CMOS image sensor to output digital bits.
CCDs were once considered the benchmark for obtaining the highest image quality in demanding applications such as medical imaging and digital photography. Compared to early CMOS image sensors, CCDs had better uniformity, and could provide greater resolution and fill factor. However, as the feature size of CMOS fabrications has been reduced, CMOS image sensors have improved to the point that they can be used in demanding imaging applications.
Photodetector and photogates in CMOS images sensors are fabricated through ion implantation, etching, deposition, etc. processing steps. CMOS image sensors are more likely than CCD images sensors to suffer from fixed-pattern and dark-current noise. CCDs also tend to have superior dynamic range. CMOS image sensors can generally be manufactured less expensively. In addition, most image-sensor support circuitry is CMOS based, so it can be integrated on the same chip as a CMOS image sensor, which lowers overall system cost and size. Also, CMOS image sensors do not require multiple voltages for readouts as do CCDs, so they typically consume only a fraction of the power of a comparable CCD image sensor. Further improvements in CMOS style sensors could have a significant positive impact on devices that make use of them.
Efforts have been directed toward the use of nanowires as photodetectors. Nanowires have been recognized as having the potential to be highly sensitive photodetectors and could represent a great advance in CMOS image sensors. However, the incorporation of nanowires as photodetectors in practical CMOS integrations has proven difficult. Additionally, the photon absorption, gain and current generation in nanowires are not fully understood.
For example, under UV illumination, it has been observed that photogenerated holes in ZnO nanowires oxidize surface oxygen species. This transient response of nanowires or how to control it is not fully understood. See, e.g., Lu et al., “Ultraviolet Photodetectors with ZnO Nanowires Prepared on ZnO:Ga/Glass Templates”, App. Phys, Lett. 89, 153101 (2006). Others have observed the oxygen sensitivity of ZnO nanowires, which may be used, for example, for gas sensing applications. See, Fan et al, “ZnO Nanowire Field Effect Transistor and Oxygen Sensing Property”, Applied Physics Letters 85, 5932 (2004).
Another issue in making practical use of nanowires as photodetectors involves the placement of nanowires and connecting into integrated circuits. While nanowires have been used in groups to realize photodetection and can be deposited in parallel by the standard Langmuir-Blodgett technique for this purpose, the registered placement and registration of nanowires necessary for complex image sensor circuits is lacking.
Fluidic assisted alignment, electrical (electrophoresis) and magnetic field guided alignment, and Langmuir-Blodgett technique, have been used previously to assemble nanowires on the surface of liquids in a well-aligned fashion, similar to nematic phase liquid crystals, and consequently transferred to the surface of solid substrates while maintaining their alignment/organization.
These techniques do not provide for the registered placement of a 1D nanostructure, however. The Langmuir-Blodgett technique as used in the art does not allow the precise control resulting nanowire location and the registration of nanowires on a substrate. Similarly, fluidic alignment cannot achieve precise control of the nanowire location and registration on a substrate, and also cannot be used with large substrates, limiting its applicability to very small scales. On the other hand, the electrophoretic technique does not work on the large scale and requires applying an electric or magnetic field to guide the nanowire assembly.
However, microbeads and nanoparticles have been placed precisely on a substrate by using either the Langmuir-Blodgett technique or self-assembly techniques combined with photolithography to predefine the pockets where the nanoparticles are going to be positioned. See Yin, et. al., J. Am. Chem. Soc. 2001, 123, 8718 and Cui, et. al. Nano Letters 4 (6); 1093-1098 (2004).